Cr(VI) removal from synthetic and real wastewaters: The use of the invasive biomass Sargassum muticum in batch and column experiments

The macroalgae Sargassum muticum was selected for the treatment of solutions containing Cr(VI). Very acidic pH values were established as optimal for Cr(VI) reduction. Algae chemical modification reduced equilibrium time to 4 h. First order kinetic model was used to describe the reduction kinetic of Cr(VI). A column experiment allowed to distinguish the processes occurring during Cr(VI) elimination: its reduction to Cr(III) and the subsequent adsorption of this species formed. Under the selected conditions the biomass was capable of reducing all the incoming Cr(VI) during 77 h. Industrial wastewaters from chrome plating industry were also tested for chromium removal.     

Effect of pH on the removal of Cr(III) and Cr(VI) from aqueous solution by modified polyethyleneimine

A novel flocculant with the capacities of reduction and chelation was prepared in this paper. The flocculant, called polyethyleneimine–sodium xanthogenate (PEX), was synthesized by modifying polyethyleneimine with carbon disulfide and sodium hydroxide. The effect of pH on the removal of Cr(III) and Cr(VI) from aqueous solution with PEX was investigated by using flocculation experiments. The results showed that in the single-ion system (only including Cr(III) or Cr(VI) in the solution), the final Cr(III) decreased with the increase in pH from 2.0 to 10.0, while the final Cr(VI) increased at first and then decreased with the increase in this pH range studied. The removal of Cr(III) was not desirable at pH lower than 7.0, whereas the final Cr(VI) concentration reached the minimum value of 0.145 mg/L at pH 2.0. In the mixture system of Cr(III) and Cr(VI), the variation tendency for the removal of Cr(III) or Cr(VI) was very similar to that obtained in the single-ion system. The oxidation–reduction potential, zeta potential, and final pH in the supernatant were also measured to analyze the above results. Furthermore, FTIR spectra revealed that dithiocarboxylic acid groups on the macromolecular chains of PEX played a major role in the Cr(VI) reduction and Cr(III) chelation.     

Photocatalytic reduction of Cr(VI) to Cr(III) in solution containing ZnO or ZSM-5 zeolite using oxalate as model organic compound in environment

Photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution containing ZnO or ZSM-5 zeolite under ambient condition was studied by using oxalate as model organic compound in the natural environment. ZSM-5 zeolite was characterized by X-ray diffraction (XRD), and point of zero net proton charge (PZNPC) titration. The effect of illumination time, mass content of catalyst (m/V), Cr(VI) initial concentrations, pH, ionic strength, and oxalate concentrations on the photocatalytic reduction of Cr(VI) was determined. The results indicate that the PZNPC of ZSM-5 zeolite is at pH 3.6 ± 0.1. At C[Cr(VI)_(initial)] = 2.00 × 10^?4 mol/L, pH 7.5 ± 0.1 and after illumination time of 24 h, the reduction of Cr(VI) were 1.1 × 10^?5 mol/L (no ZSM-5 zeolite, 4.0 × 10^?3 mol/L oxalate) and 1.0 × 10^?5 mol/L (0.4 g/L ZSM-5 zeolite, no oxalate), respectively; whereas the reduction of Cr(VI) achieved 1.0 × 10^?4 mol/L in the presence of 0.4 g/L ZSM-5 zeolite and 4.0 × 10^?3 mol/L oxalate. The removal of Cr(VI) from solution is dependent on pH value. The results are important for the application of zeolites in the treatment of Cr(VI) polluted solution in the natural environment.     

Adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin

Series of resin selection experiments were carried out and the KIP210 strong base anion exchange resin was confirmed to have the maximum equilibrium adsorption capacity to remove Cr(VI) from wastewater. The adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin were investigated completely and systematically. The static experiments were performed to study the effects of various parameters, such as shaking speed, resin dosage and pH during the adsorption process. The results indicate that the effect of external diffusion is eliminated at 160 rpm, the best pH value is 3.0 and the removal percentage of Cr(VI) increases with the increase of the resin dosage. The adsorption of Cr(VI) on KIP210 agrees well with the Langmuir isotherm and the adsorption parameters of thermodynamics are ΔH = 26.5 kJ mol⁻¹, ΔS = 126.7 J mol⁻¹ K⁻¹ and ΔG < 0. It demonstrates that the adsorption of Cr(VI) on KIP210 is a spontaneously endothermic physisorption process. Moreover, the adsorption process can be described well by a pseudo-second-order kinetic model and the activation energy is 30.9 kJ mol⁻¹. The kinetic analysis showed that the adsorption rate is controlled by intraparticle diffusion. The resin is successfully regenerated using the NaOH solutions.     

Kinetics,equilibrium and thermodynamic studies on biosorption of hexavalent chromium by dead fungal biomass of marine Aspergillus niger

Quite a number of reports are available on metal binding capacity of different groups of microorganisms. However, reports on the equilibrium studies on biosorption by marine fungi are quite inadequate. The present study was carried out in a batch system using dead biomass of marine Aspergillus niger for the sorption of Cr(VI). The removal rate of Cr(VI) was increased with a decrease in pH and an increase in Cr(VI) and biomass concentration. A. niger exhibited the highest Cr(VI) uptake of 117.33 mg g^?1 of biomass at pH 1.0 in the presence of 400 mg l^?1 Cr at 50 °C. Kinetics studies based on fractional power, zero order, first order, pseudo-first order, Elovich, second order and pseudo-second order rate expressions have also been carried out. The experimental data were analyzed using five, two-parameter isotherms (Langmuir, Freundlich, Dubinin–Radushkevich, Temkin and Halsey). It was observed that Langmuir model exhibited the best fit to experimental data. Thermodynamic parameters of the biosorption (ΔG°, ΔH° and ΔS°) were also determined.     

Biomass assisted microfiltration of chromium(VI) using Baker's yeast by ceramic membrane prepared from low cost raw materials

This work deals with the preparation of ceramic microfiltration membrane from inexpensive raw materials such as kaolin, quartz, calcium carbonate by uniaxial dry compaction method. The prepared green membrane was initially dried at 100 °C for 24 h, 200 °C for 24 h and finally sintered at 900 °C for 6 h. The properties of the membrane such as porosity, flexural strength, chemical stability and hydraulic permeability were investigated. The fabricated membrane possessed an average pore diameter of 1.32 μm, porosity of 30% and flexural strength of 34 MPa. Furthermore, the chemical stability of the membrane was found to be excellent. Eventually, the separation performance of the membrane in terms of flux and removal of chromium(VI) ion using baker's yeast biomass as a function of applied pressure, pH, metal ion concentration and biomass dosage was also studied. The removal of Cr(VI) was found to be strongly dependent on the initial pH of the solution. At lower pH, the metal solution shows higher removal due to higher binding of the metal ion with biomass. It was also observed that the removal of Cr(VI) ion increases with increasing the biomass concentration and decreases with increasing the metal ion concentration. The removal of Cr(VI) was found to be independent of the applied pressure. The maximum removal of Cr(VI) was found to be 94% with the permeate flux of 2.07 × 10^-5 (m³/m² s) for a metal solution concentration of 100 mg/L.     

Synthesis and characterization of robust zero valent iron/mesoporous carbon composites and their applications in arsenic removal

Nanoscale zero valent iron particles (nZVI) have been developed by in situ reduction of Fe³⁺ ions onto a mesoporous type of carbon matrix – Starch-Derived Mesoporous Carbonaceous Material previously reported and marketed commercially as “Starbon”. The obtained nZVI/Starbon hybrid material exhibits homogeneous distribution of nZVI (10–20 nm) within the carbon matrix, surface area of 141 m²/g and a total iron loading of 1 mmol per gram of the composite, in accordance with transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption–desorption measurements, Infrared (IR)/Raman spectroscopy and Thermal gravimetric (TG)/Differential Thermal analysis (DTA). Electron Paramagnetic Resonance (EPR) and proton binding measurements show that the nanoparticles have a core–shell structure with iron(III) oxide/hydroxide shell due to partial air-oxidation of nZVI and the composite exhibits four different types of proton binding groups. Most importantly, the nZVI/Starbon hybrid has been tested as absorbent for As(III) removal showing a total removal of 358 μmol (26.8 mg) of As(III) per gram of the composite at pH = 7. We also discuss the principal role of surface OH groups of iron oxide in arsenic uptake and the crucial effect of pH on removal efficiency.     

Adsorption performance of suitable nanostructured novel composite adsorbent of poly(N-methylaniline) for removal of heavy metal from aqueous solutions

Poly(N-methylaniline) (PNMA) composite composed of N-methylaniline and poly(ethanol) was prepared by in situ polymerization technique and characterized using FTIR, SEM-EDS and XRD instruments. Characterization of product revealed that this composite is crystalline in nature and the particles size is less than100 nm. The potential of this composite in removal of Cr(VI) ions from synthetic aqueous effluents was investigated by batch sorption system. The experimental results confirmed that this adsorbent has the potential application for removal of Cr(VI) ions from aqueous solution with the sorption capacity of 125 mg/g of Cr(VI)/0.1 g of adsorbent.     

Synthesis,characterization of Fe3O4@glycine doped polypyrrole magnetic nanocomposites and their potential performance to remove toxic Cr(VI)

Fe₃O₄ coated glycine doped polypyrrole magnetic nanocomposite (Fe₃O₄@gly-PPy NC) was prepared via coating of suspended Fe₃O₄ nanoparticles with gly-PPy. FE-SEM and HR-TEM images indicated that Fe₃O₄ nanoparticles were encapsulated by precipitating gly-PPy moieties. Chromium(VI) adsorption followed a Langmuir isotherm with maximum capacity of 238–303 mg/g for a temperature range of 25–45 °C at pH 2. The adsorption process was governed by the ionic interaction and the reduction of Cr(VI) to Cr(III) by the PPy moiety. Results showed that NCs are effective adsorbents for the removal of Cr(VI) from wastewater and can be separated by external magnetic field from the reactor.     

Poly(N,N-dimethylaminoethyl methacrylate)/graphene oxide hybrid hydrogels: pH and temperature sensitivities and Cr(VI) adsorption

Different amounts of graphene oxide (GO) were incorporated to N,N-dimethylaminoethyl methacrylate (DMAEMA), fabricating a series of pH and temperature dual sensitive PDMAEMA/GO hybrid hydrogels by in situ polymerization. Their microscopic network structures as well as swelling properties and Cr(VI) adsorption were characterized. The equilibrium swelling ratios (ESR) of hydrogels increased significantly with 0.5 wt% GO feeding of DMAEMA amount, and then decreased with further GO loading increasing. All hydrogels showed obvious deswelling when pH value of swelling mediums increased from 5 to 10 gradually. At pH 7, hydrogels revealed slight ESR increment with temperature up to 50 °C, above which obvious deswelling occurred. In pH 8 buffer, 0.5 wt% of GO loading triggered lower critical solution temperature (LCST) to decrease by 3 °C, and 2–7 °C increment was observed when 1–6 wt% of GO was loaded, as compared with that of GO-free PDMAEMA hydrogel. Cr(VI) adsorption of hydrogels was also improved by the introduction of GO to some extent, and the maximum Cr(VI) adsorption of 180 mg/g was realized, indicating that the obtained PDMAEMA/GO hybrid hydrogels possess excellent adsorption performance.     

Adsorption of hexavalent chromium from aqueous solutions by bio-chars obtained during biomass pyrolysis

In this study, bio-chars were evaluated as a potential adsorbent for the removal of Cr (VI) ions from aqueous solutions. The effects of some important parameters including initial pH (1.5–7), adsorbent dose (0.2–5 g/L), contact time (5–900 min) and initial Cr (VI) ion concentration (5–75 mg/L) were tested on the removal of Cr (VI) ions from aqueous solution in batch experiments. Maximum adsorption capacities of the tested bio-chars under the certain experimental conditions determined as optimal were 3.53 mg/g for NCBC, 3.97 mg/g for NZCBC and 6.08 mg/g for ACBC, respectively. Results of the kinetic and isotherm modeling studies revealed that the adsorption data fitted well with a pseudo-second order and Langmuir model. In among the tested bio-chars, the bio-char (ACBC) was largely equivalent to activated carbon: AC (9.97 mg/g) in terms of adsorption capacity. All results indicated that the bio-chars had higher adsorption capacity than some chars and activated carbons reported previously, and also that these bio-chars could be used successfully as low-cost adsorbents for the removal of chromium ions from aqueous solutions under the tested experimental conditions.     

SnS2/TiO2 nanocomposites with enhanced visible light-driven photoreduction of aqueous Cr(VI)

SnS₂/TiO₂ nanocomposites have been synthesized via microwave assisted hydrothermal treatment of tetrabutyl titanate in the presence of SnS₂ nanoplates in the solvent of ethanol at 160 °C for 1 h. The physical and chemical properties of SnS₂/TiO₂ were studied by XRD, FESEM, EDS, TEM, XPS and UV–vis diffuse reflectance spectra (DRS). The photocatalytic activity of SnS₂/TiO₂ nanocomposites were evaluated by photoreduction of aqueous Cr(VI) under visible light (λ>420 nm) irradiation. The experimental results showed that the SnS₂/TiO₂ nanocomposites exhibited excellent reduction efficiency of Cr(VI) (~87%) than that of pure TiO₂ and SnS₂. The SnS₂/TiO₂ nanocomposites were expected to be a promising candidate as effective photocatalysts in the treatment of Cr(VI) wastewater.     

Cationic polymer chain tethered on the pore-wall of 3-D ordered macroporous resin for the removal of hexavalent chromium from aqueous solution

A novel 3-D ordered macroporous (3DOM) adsorbent with a cationic polymer chain (poly(N,N-dimethylaminoethyl methacrylate), PDMAEMA) tethered on the pore wall was prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) for the removal of toxic Cr(VI) ions from aqueous solution. In comparison with recently reported adsorbents, the adsorbent remarkably stands out owing to large adsorption capacity, relatively fast kinetics, and high stability in the regeneration process. The adsorption capacity significantly depended on the solution pH and there was a wide working pH range that is much convenient in practical application. Kinetics of Cr(VI) adsorption by the 3DOM adsorbent was studied in batch experiments, in the temperature range 298–318 K. The equilibriums were arrived within 120–130 min and a pseudo-second order model can be described well. In the adsorption isotherm study, experimental data followed the Langmuir adsorption model. The maximum adsorption capacity increased with the increase of temperature, and reached the high value of 431.0 mg/g at 308 K. Thermodynamic parameters revealed spontaneous and endothermic adsorption processes. Furthermore, the 3DOM adsorbent remained high adsorption capacity (above 90% of the original Cr(VI) loading capacity) after 15 adsorption–desorption cycles by simply using sodium hydroxide solution as the desorption liquid, which ensured the reusability of 3DOM adsorbent.     

Combined UHV and ambient pressure studies of 1,3-butadiene adsorption and reaction on Pd(1 1 1) by GC,IRAS and XPS

The hydrogenation of 1,3-butadiene on Pd(1 1 1) at 300 K was studied at atmospheric pressure by infrared reflection absorption spectroscopy (IRAS) and gas chromatography (GC). Kinetic measurements showed 1-butene, trans-2-butene and cis-2-butene as primary products. Once 1,3-butadiene had been completely consumed, 1-butene was re-adsorbed on the surface producing trans-/cis-2-butene through isomerization and n-butane through hydrogenation. These results were corroborated by in situ IRAS spectroscopy. Post-reaction analysis by X-ray photoelectron spectroscopy (XPS) in the C1s region revealed a band at 284.2 eV, corresponding to adsorbed butadiene and/or carbonaceous deposits. Quantification of this peak revealed a total carbon coverage of 0.3 ML. Nevertheless, deactivation due to carbon deposition was a minor effect under our reaction conditions, as indicated by the kinetics of the subsequent butene hydrogenation reaction. Temperature-dependent XPS experiments after butadiene adsorption at 100 K indicated a high stability of the diene molecule with hardly any desorption and/or decomposition up to 500 K. Above this temperature, butadiene decomposed to carbon species that eventually dissolved in the Pd bulk above 700 K.     

Synthesis,characterization and kinetics properties of chromium(III) complex [Cr(3-HNA)(en)2]Cl · H2O · CH3OH

The reaction of chromium(III) chloride, 3-hydroxy-2-naphthoic acid (3-HNA) and ethylenediamine (en) led to the formation of complex [Cr(3-HNA)(en)₂]Cl · H₂O · CH₃OH, Bis(ethylenediamine-κ²N,N′)(3-hydroxy-2-naphthoic acid-κ²O,O′) chromium(III) monochloride monohydrate monomethanol. The kinetics of transfer of Cr(III) from the title compound to the low-molecular-mass chelator EDTA and to the iron-binding protein apoovotransferrin (apoOTf) were carried out by means of UV–Visible (UV–Vis) and fluorescence spectra in 0.01 M Hepes at pH 7.4. The second-order rate constants were calculated, respectively. The results show that Cr(III) can be transferred from the complex to apoovotransferrin.     

Characterization of the removal of Chromium(VI) from groundwater by electrocoagulation

A batch electrocoagulation system has been evaluated for the removal of Cr(VI) from brackish groundwater under different operating conditions. The influence of electrode type, applied current density, initial pH, initial chromium concentration, conductivity and temperature were evaluated. The experimental results indicated that chromium removal increased with increasing the applied current density and conductivity. The efficiency of different electrode arrangements (iron, aluminum) was also assessed, and indicated that Fe–Fe electrode pair was the most efficient arrangement and was able to achieve 100% Cr removal at an electrocoagulation time of 5 min, a current density of 7.94 mA/cm², and pH of 8 at room temperature 25 °C. The generated sludge for the iron electrodes was characterized using EDS, X-ray fluorescence (XRF) and FE-SEM. The analysis confirmed the formation and precipitation of Fe(OH)₃ and Cr(OH)₃ as solids. Overall, the study affirmed that electrocoagulation is a reliable technique for the purification of groundwater with an estimated energy consumption of 0.6 kWh/m³.     

Chromium (VI) removal by methylated biomass of Spirulina platensis: The effect of methylation process

Biosorption of heavy metals is an interesting approach to treat industrial wastewaters by an environmentally friendly system. Spirulina platensis biomass, an effective biosorbent for cations, cannot be used to adsorb chromate due to its negatively charged surface close to neutral conditions; therefore, methylation of biomass was performed to increase its adsorption capacity under these conditions. Batch adsorption tests carried out varying both Cr(VI) and methylated biomass concentrations showed that 2–4 g l^?1 of biosorbent were able to remove Cr(VI) with efficiency ≥80%, while higher Cr(VI) levels (43–50 mg l^?1) showed low removal efficiency. The model of Langmuir was shown to describe the adsorption phenomenon better than the Freundlich one. The values of the overall adsorption capacity of methylated biomass suggested that increased biosorbent availability does not necessarily correspond to larger amount of adsorbed metal. FT-IR spectra of dried and methylated biomass of S. platensis allowed us monitoring the efficiency of the methylation process through the analysis of CH and COO^? vibrational stretching modes, taken as diagnostic of this process.     

Effect of strontium and cerium doping on the structural characteristics and catalytic activity for C3H6 combustion of perovskite LaCrO3 prepared by sol–gel

Two series of Sr- or Ce-doped La_1−xM_xCrO₃ (x = 0.0, 0.1, 0.2 and 0.3) catalysts were prepared by thermal decomposition of amorphous citrate precursors followed by annealing at 800 °C in air atmosphere. The effect of Ce and Sr on the morphological/structural properties of LaCrO₃ was investigated by means of thermogravimetric/differential thermal analysis (TG/DTA) of the precursors decomposition under air, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), transmission electron microscopy–X-ray energy dispersive spectroscopy (TEM–XEDS), S_BET determination, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results are employed to explain catalytic activity results for C₃H₆ combustion. It is shown that the lanthanum chromite perovskite structure is obtained upon thermal treatment of the sol–gel derived precursors at T > ca. 800 °C. The presence of the dopant generally induces the formation of segregated oxide phases in the samples calcined at 800 °C although some introduction of the Sr in the perovskite structure is inferred from EPR measurements. The oxidation activity becomes maximised upon formation of such doped perovskite structure.     

Fabrication of copper coated polymer foam and their application for hexavalent chromium removal

The polymer foam coated with zero-valent copper (Cu⁰) was designed and prepared for the removal of hexavalent chromium (Cr(VI)) in water. Firstly, porous poly(tert-butyl acrylate) was fabricated by concentrated emulsion polymerization and then acrylic acid groups were generated on the surface of foam by hydrolysis reaction. Secondly, with the help of the large amount reactive carboxylic acid groups, polyethyleneimine (PEI) were chemically grafted onto the surface by the reaction between amine group and acrylic acid group. Finally, zero-valent copper was reduced by sodium borohydride (NaBH₄) and coated on the surface of polymer foam. Thus the copper functionalized porous adsorbent (Cu⁰–PEI–PAA) was constructed, and then applied for removing Cr(VI) from aqueous solution. The removal mechanism of Cr(VI) involved redox reaction by zero-valent copper and adsorption by amine groups, simultaneously. As a result, 99.5% of Cr(VI) could be removed within 2 h, and the maximum removal capacity for Cr(VI) of Cu⁰–PEI(1800)–PAA was 9.16 mg/g. Furthermore, the effect of initial concentration of Cr(VI), pH value, and temperature on the Cr(VI) removal was investigated. Therefore, the as-prepared zero-valent copper-loaded polymer foam could be an efficient and promising remediation material to remove Cr(VI) from wastewater.     

Removal of As(V) and Cr(VI) in aqueous solution by sand media simultaneously coated with Fe and Mn oxides

In this research, sand media simultaneously coated with iron and manganese (iron and manganese coated sand, IMCS) were applied to treat synthetic wastewater contaminated with both Cr(VI) and As(V). Scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD) were used to characterize the surface properties of the coated layer of IMCS. Adsorption of Cr(VI) and As(V) onto IMCS followed a typical anionic type, showing a gradual decrease of adsorption as the solution pH increased. In a single system, IMCS showed a greater adsorption capacity for As(V) than that for Cr(VI) over the entire solution pH range. In a binary system, As(V) preferentially occupied the limited adsorption sites on IMCS and therefore Cr(VI) adsorption was suppressed. This result indicates that As(V) adsorption onto the surface of IMCS occurs through a strong chemical bonding such as an inner-sphere complex. As(V) adsorption onto the IMCS was well described by second-order kinetics. From the adsorption isotherm experiments at pH 4, the maximum adsorbed amount of Cr(VI) and As(V) onto IMCS in a single system was 102 mg/kg and 455 mg/kg, respectively.